1. Field of the Invention
This invention relates generally to power conversion and generation systems and particularly to distortion compensation algorithms.
2. Description of the Prior Art
Power electronic inverter bridges are widely used to convert power between Ac and DC circuits. To match the AC and DC circuits, the inverter bridge has generally one leg for each phase. The legs are made up of two electronic switches. One switch is connected to the positive DC line and the AC line. The other switch is connected to the negative DC line and the AG line. Using the two switches, the AC line is alternately connected to the positive and negative De lines. In this way, power transfer can be achieved.
When both switches are on at the same time, the positive and negative DC lines become connected, causing a high current, which can be detrimental to the circuit, to flow. It is thus very important that both switches never be on at the same time. To prevent this situation, a xe2x80x9cdead-timexe2x80x9d is inserted between the turning off of one switch and the turning on of the other switch. Unfortunately, this dead-time and other non-linearities of the power converter cause distortion of the current and/or voltage in the AC circuit.
Prior art solutions for reducing AC current and/or voltage distortion focus on using high bandwidth feedback control, which suffers from poor rejection of noise on the measurement of current/voltage signals. Also, high bandwidth feedback control is inflexible when realized using analog circuits and expensive when realized using digital circuits. If low bandwidth feedback control is employed to increase flexibility or reduce cost then significant distortion of the AC current/voltage can occur. Some applications can tolerate the resulting distortion in the AC current/voltage, but will suffer a loss of efficiency. Where the AC circuit is the utility grid, the resulting distortion in the AC current/voltage may exceed the appropriate standards. Many of the same problems are present where a power converter is used to connect between two AC circuits with distortion present in either or both of the AC circuits.
What is needed therefore is an effective apparatus and method for reducing harmonic distortion in AC circuits.
The present invention provides, in a first aspect, a method for reducing distortion in an electrical quantity in a circuit, including the steps of measuring values of the electrical quantity, sampling the electrical quantity at fixed instants in time, determining a phase angle of the circuit at each sampling, storing historical information about differences between the measured values and corresponding reference values of the electrical quantity throughout a cycle of the electrical quantity, and applying feedback control based upon the historical information and the phase angle for forcing the measured electrical quantities to converge towards the reference electrical quantities.
The present invention provides, in a second aspect, an apparatus for reducing distortion in an electrical quantity in a circuit, including means for measuring values of the electrical quantity, means for sampling the electrical quantity at fixed instants in time; means for determining a phase angle of the circuit at each sampling, a memory for storing historical information about differences between the measured values and corresponding reference values of the electrical quantity throughout a cycle of the electrical quantity, and means for applying feedback control based upon the historical information and the phase angle for forcing the measured electrical quantities to converge towards the reference electrical quantities.
The present invention provides, in a third aspect, a method for reducing distortion in an electrical quantity in a circuit, including the steps of measuring values of the electrical quantity, sampling the electrical quantity at fixed instants in time, determining a phase angle of the circuit at each sampling, storing historical information about differences between the measured values and corresponding reference values of the electrical quantity throughout a cycle of the electrical quantity, allocating memory locations capable of storing a fixed number of values to cover all of the cycle, and applying feedback control based upon the historical information and the phase angle for forcing the measured electrical quantities to converge towards the reference electrical quantities.
The present invention provides, in a fourth aspect, an apparatus for reducing distortion in an electrical quantity in a circuit, including means for measuring values of the electrical quantity, means for sampling the electrical quantity at fixed instants in time, means for determining a phase angle of the circuit at each sampling, means for storing historical information about differences between the measured values and corresponding reference values of the electrical quantity throughout a cycle of the electrical quantity, means for allocating memory locations to cover all of the cycle, wherein the memory locations are capable of storing a fixed number of values, and means for applying feedback control based upon the historical information and the phase angle for forcing the measured electrical quantities to converge towards the reference electrical quantities.
The present invention provides, in a fifth aspect, a method for reducing harmonic distortion in a current/voltage of an AC circuit of a power converter bridge operating under digital current/voltage control, including the steps of measuring values of the AC circuit current/voltage, sampling the AC circuit current/voltage at fixed instants in time, determining a phase angle of the circuit at each sampling, storing historical information about differences between the measured values and corresponding reference values of the AC current/voltage throughout a cycle of the AC circuit current/voltage, allocating memory locations to cover all of the cycle, wherein the memory locations are capable of storing a fixed number of values and applying feedback control based upon the historical information and the phase angle for forcing the measured currents/voltages to converge towards the reference currents/voltages.
Finally, the present invention provides-an apparatus for reducing distortion in an AC circuit current/voltage in a circuit, including means for measuring values of the AC circuit current/voltage, means for sampling the AC circuit current/voltage at fixed instants in time, means for determining a phase angle of the circuit at each sampling, means for storing historical information about differences between the measured values and corresponding reference values of the AC current/voltage throughout a cycle of the AC circuit current/voltage, means for allocating memory locations to cover all of the cycle, wherein the memory locations are capable of storing a fixed number of values, and means for applying feedback control based upon the historical information and the phase angle for forcing the measured currents/voltages to converge towards the reference currents/voltages.